At Wed, 13 Oct 2010 16:39:18 -0700, Smith, Jacob N wrote:
I've only had a chance to read the Austern paper "Segmented Iterators and Hierarchical Algorithms" once. I agree that the segmented iterators are rather appealing as an interface to packs-of-pods-datatype. However, I'm not sure the conceptual interface to segmented iterators are rich enough. Luke Simonson has more experience with the construction of an iterator pattern around SIMDs used in a high-performance system than I do. However I seem I to remember that the pattern for SIMD iteration always looked like this:
Loop A? Loop B* Loop C?
Where Loops A & C handle the unaligned start/end (if they exist) and Loop B* handles (0 or more of) the aligned packs of the sequence. Handling the edge-cases for A & C were definitely not the same --- there are memory exceptions that can occur at the end-of-range for Loop C that can't (or usually don't) occur for Loop A when doing unaligned loads/stores. I seem to remember that it was insufficient to just mark "not Loop B" but it was necessary to explicitly mark foreach operations as "Loop A" vs. "Loop C". It seems that this division of Loop A/B/C will need to be exposed through the iterator.
I think you should read the paper again and look at the implementation of one of the algorithms there. They all have that structure. -- Dave Abrahams BoostPro Computing http://www.boostpro.com